1. Field of the Invention
The present invention relates generally to an underground mine prop for supporting the roof, and, more particularly, to a yieldable mine prop that provides controlled yielding along a length thereof while preventing buckling failure of the mine prop when subjected to longitudinal forces.
2. Description of the Prior Art
Over the past several years, Burrell Mining Products, Inc. of New Kensington, Pa. has successfully marketed and sold a mine roof support product sold under the trademark THE CAN®.” THE CAN support is comprised of an elongate metal shell that is filled with aerated concrete. The use of aerated concrete in THE CAN support allows the support to yield axially in a controlled manner that prevents sudden collapse of an underground mine roof.
THE CAN support has a height to width ratio (i.e., slenderness ratio) that prevents the support from buckling along its axial length. The slenderness ratio of a column having a circular cross-section is defined by the length of the column divided by the radius. Because THE CAN has a typical slenderness ratio of between about 5 and 10 for most sizes of THE CAN, THE CAN yields axially before it buckles or kneels. As such, THE CAN support yields axially as the aerated concrete within the product is crushed and maintains support of a load as it yields.
A typical size of THE CAN support is approximately six feet (1.8 meters) in height and two feet (0.6 meters) in diameter. This results in approximately 18.85 cubic feet (0.51 cubic meters) of aerated concrete contained within each support. As such, even using aerated concrete, the weight of the aerated concrete and its associated metal shell results in a product that typically requires various machinery, such as a fork lift, to move each support. In addition, the general sizes of THE CAN supports somewhat limits its use to certain mine applications, such as longwall mining operations where the size of THE CAN support does not interfere with the mining operations. While being extremely successful in those mines that can utilize THE CAN support, there still exists a need in the industry to provide a mine prop that has potential applicability to every underground mining operation, or tunnel type environment for that matter, that can be carried by hand by the user.
By contrast, an oak wood post having a length of 6.5 feet and a diameter of 6 inches will have a slenderness ratio of 26. Such a post will have a maximum axial load handling capability (assuming that the load is not applied eccentrically) of about 16,000 lbs. For a post formed from spruce, the maximum safe axial load handling capability for a post that is 6.5 feet in length and 6 inches in diameter is about 13,600 pounds. In addition, when a wood post yields by kneeling or buckling, such yielding will result in catastrophic failure of the post in which the post can no longer support the load.
Because of the obvious problem associated with such catastrophic failure of posts, various mine props have been developed in the art for supporting the roof of an underground mine. Such mine props have included, for example various configurations of wood beams encased in metal housings, and complex hydraulically controlled prop devices. Such props, however, do not allow for controlled axial yielding while preventing sideways buckling or kneeling in a simple, lightweight prop that can be hand carried by a user.
Thus, it would be advantageous to provide a mine prop that is relatively lightweight so that it can be hand carried to a desired location, that can yield upon itself without kneeling or buckling, is relatively easy to manufacture and cost effective, and can be utilized in virtually any underground mining situation where such a prop may be desired.